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Nature sure has created some tiny monsters when it comes to microbes—bugs that make us throw up, ooze pus, bleed out of our eyes and cough up blood. But which take top honors (or dishonors) for being the most lethal of all?

HIV/AIDS, the Global Tragedy

The Human Immunodeficiency Virus or HIV causes the disease we know as AIDS. AIDS is a pandemic—a disease outbreak that is happening all over the world. In 2000 alone, 3,000,000 people died of AIDS. Since the AIDS pandemic began around 1985, almost 22,000,000 people have died from the disease. Right now, HIV infects more than 36,000,000 people across the world, so that death toll will definitely get a lot bigger over the next several years.

HIV doesn’t kill people itself. Instead, the virus shuts down a person’s immune defenses—the tools used to fight off invading germs—by infecting and destroying important immune cells called T cells. Once a person loses too many T cells, his or her body can no longer deal with other microbes that cause infections. HIV merely opens the floodgates. Eventually HIV-infected people become overrun by germs and die of lung infections, skin infections or other diseases.

We now have several drugs that fight HIV. They cannot cure the infection, but they can keep it in check. Unfortunately, victims of HIV have to take several of these drugs every day for life and the drugs are very expensive. In Africa and other developing parts of the world where HIV is spreading most rapidly, most people don’t have enough money to buy these drugs. That’s why we can expect the death toll from HIV/AIDS to get a lot bigger over the next several years.

The Flu Virus—Even Deadlier than HIV?

Oh come on now, I’m sure you’re saying. The flu virus here in a list of the most lethal microbes?

Sure, every year during what’s called "the flu season" tens of thousands of people get the flu. Despite feeling all achy and lousy for several days, most people eventually beat the virus and recover just fine.

But that’s not how it went during the 1918 flu season. That year, World War I was raging so the effects of the flu virus sort of got overshadowed by the bigger, more obvious effects of bullets and bombs.

However, in 1918 the flu virus killed at least 675,000 American people. Worldwide, it killed at least 21,000,000 people. These are low estimates. Some research now suggests that the flu may have killed 40,000,000 to 50,000,000 people around the world that year.

Researchers still aren’t sure why the 1918 flu virus was so deadly. Victims of the flu that year died in a gruesome way, the virus causing so much fluid to build up in their lungs so rapidly that it was like drowning.

If higher estimates of how many people died from the 1918 flu are true, then the 1918 flu is the deadliest microbe ever in a single year. Thank goodness we haven’t seen a flu bug that deadly again—but will we someday?

The Plague

The 1918 flu virus and HIV are the biggest killers of modern times. But back in the 14th century, the bacterium that causes bubonic plague, or the Black Death as it was also known, was the baddest bug of all.

In just a few years, from 1347 to 1351, the plague killed off about 75,000,000 people worldwide, including one-third of the entire population of Europe at that time. It spread through Asia, Italy, North Africa, Spain, Normandy, Switzerland, and eastward into Hungary. After a brief break, it crossed into England, Scotland, and then to Norway, Sweden, Denmark, Iceland and Greenland.

The plague bacterium is called Yersinia <yer-sin-ee-uh> pestis. There are two main forms of the disease. In the bubonic <boo-bah-nick> form, the bacteria cause painful swellings as large as an orange to form in the armpits, neck and groin. These swellings, or buboes, often burst open, oozing blood and pus. Blood vessels leak blood that puddles under the skin, giving the skin a blackened look. That’s why the disease became known as the Black Death. At least half of its victims die within a week. The pneumonic <new-mon-ick> form of plague causes victims to sweat heavily and cough up blood that starts filling their lungs. Almost no one survived it during the plague years.

Yersinia pestis is the deadliest microbe we’ve ever known, although HIV might catch up to it. Yersinia pestis is still around in the world. Fortunately, with bacteria-killing antibiotics and measures to control the pests—rats and mice—that spread the bacteria, we’ve managed to conquer this killer.

Ebola, the Bloody Virus

Some of you at the beginning of this page might have thought of the Ebola virus as the scariest, deadliest microbe you’ve ever heard of. Those of you who did so have probably seen the movie Outbreak or read The Hot Zone, which spotlighted this grisly virus.

Ebola is definitely a nasty killer. It is part of a group of viruses that, among other effects on the body, cause the blood to stop clotting. Victims begin oozing blood from their mouths, noses, internal organs, even their eyes. It kills up to almost 90% of those who get infected.

With that kind of death rate, Ebola would be the deadliest microbe of all if it was more common. Fortunately, infections by this virus are pretty rare. There have only been seven outbreaks in humans. It has killed just over 800 people since the first outbreak in the Democratic Republic of the Congo (formerly called Zaire) in 1976.

While outbreaks of this virus are rare and relatively small, Ebola is still one mean microbe.